This invention relates to the field of voltage regulators for use with automobile battery charging systems and, more particularly, to a means of providing more accurate regulation when the system is operating at or near full load.
Battery charging systems should, ideally, supply a constant voltage, regardless of the load required. In electronic voltage regulators this is accomplished by switching on and off the field or excitation current which produces the electromagnetic field of the alternator. The switching control signal is produced by a comparator which compares a ramp, which may or not be stepped, with the alternator output signal. As the alternator load increases, the duty cycle of the regulator output signal can become 100%, causing the control pulses for the excitation to be entirely skipped. When control pulses are skipped, the average voltage output is reduced, and if the switching rate is too low, flicker of the automobile lamps may be objectionable, and other electronic devices may likewise be affected. In one prior art system, a "fixed frequency" voltage regulator having a variable duty cycle was provided, the "fixed frequency" referring to the clock signal Which controls the "stairstep" modulation on the reference voltage of the regulator.